A recent accident in June 2009 (Rio-Paris flight AF 447) highlighted the problem of searching in a limited time for “black boxes” (onboard flight recorders) submerged following the destruction of the aircraft above the sea. This search was done by detection, using a submerged sensor, of the signals transmitted by the “pingers” (transmitters) of these black boxes. Such signals consist of a carrier, theoretically sinusoidal (the oscillator producing it is not very stable) chopped in a substantially periodical manner (nor is the chopping very stable). The result is a series of “bursts” of substantially sinusoidal oscillations with a rectangular envelope. Signals of this type are also encountered in the reception of sonars or of radar signals.
By way of illustration, the conventional “pingers” of the onboard flight recorders used in aeronautics will be described here. These “pingers” are small transmitters which transmits, for a duration of around 30 days, acoustic signals with a sinusoidal carrier (with a frequency of 37.5 KHz in the case of the black boxes of AF447), the duration of which is of the order of 10 ms, at a repetition rate of the order of a second, in order to be as sparing as possible with the power supply source of the pingers. These pingers start working on contact with the water. The transmitted signal (a series that is uninterrupted for 30 days) is overlaid, on the receiver used, with the acoustic ambient noise which may be the ocean noise or the specific noise of the search platform including the receiver.
FIG. 1 diagrammatically shows an exemplary timing diagram of two bursts or successive periods 1, 2 of sinusoids with a rectangular envelope transmitted by a “pinger”. In the drawing, these two periods are shown close together, but in reality, because the useful period (during which the sinusoids are transmitted) is very small compared to the chopping repetition period (of the order of 1%, for the example cited above), the two successive periods are widely separated from one another.
The current systems for searching for the “black boxes” essentially use sonogram audio (after heterodyning and filtering) and video, and are systems designed to be used in real time by an operator. The detection efficiency of these known systems for searching for the “black boxes” is not always sufficient, particularly in a choppy marine environment. Correlatively, the current approach requires the constant attention of several trained operators for extremely long periods (up to 30 days). Moreover, the playing back of the audio recordings made demands as much time as their acquisition (because it is based on listening).